I. 



GENERAL MORPHOLOGY 



A. EXTERNAL APPEARANCE OF THE ANIMAL 



CONTRARY TO GENERAL CUSTOM, the characteristics of the 

 animal and its gross anatomy will be considered before taking up 

 the shell and its variations. This procedure seems more logical 

 since it is the animal which secretes and builds up the shell or calcareous 

 covering (see plate 70, fig. 1). 



The head and foot protrude from the shell and bear several important 

 organs. The foot is long and variably narrow, broadly rounded in front, 

 and usually tapering to a point behind. There is a large velar area above 

 and in front of the foot which is retained from the embryonic animal, as in 

 Lyinnaca, and bears the head, eyes, and tentacles. This velum is broad and 

 short and is capable of great extension and retraction; it is usually some- 

 what semicircular in shape. 



The tentacles are very long and filiform, varying from one-half to the 

 whole length of the foot, and are placed at about the junction of the velum 

 with the foot. The eyes are placed on swellings at the inner base of the 

 tentacles. The mantle collar does not extend beyond the edge of the aper- 

 ture of the shell. The texture of the foot is smooth, not granular as in 

 some of the land snails, and there are no grooves on the body, the animals 

 resembling the Lymnaeas in this respect. 



The Planorbidae differ from the related family Lymnaeidae in having 

 certain of the organs on the left side (sinistral), the organs in Lymnaeidae 

 being on the right side (dextral). In this respect the Planorbidae resemble 

 the families Physidae and Bulinidae. The tentacles of the Planorbidae, 

 Physidae, and Bulinidae are also similar and differ from the Lymnaeidae 

 and Ancylidae, in which the tentacles are flattened and triangular. 



The breathing organ of the Planorbidae, a modified lung as in the family 

 Lymnaeidae, is supplemented by a prolongation of the anal region (auri- 

 form lobe), called the pseudobranch or false gill, which may take over the 

 function of breathing when the animal is submerged and can not obtain 

 free air. This organ is highly vascular, many blood vessels entering it from 

 the lower part of the lung cavity. It is comparatively a recent organ, 

 being post-natal in origin, not having been observed in embryological 

 development. 



The pseudobranch varies greatly in shape, being long and narrow, wide, 

 leaf-like, or rounded in different genera. It always bears a portion of the 

 rectum and the anus opens at the upper right side. There is usually a ridge, 

 more or less heavy and distinct, which begins on the upper part of 

 the rectum and ends in the middle or at the base of the pseudobranch. The 

 variations of the pseudobranch are shown on the plates illustrating the anat- 

 omy of each genus. A siphon-like extension of the mantle, called the 

 pncumostome or respiratory opening, which may be greatly expanded, 

 jirovides a tube-like conduit for free air to reach the lung. The jiseudo- 

 branch is on the left side of the body and the pneumostome is to the right 

 of this organ nearer the median line of the neck. 



